It could well be that producers roll off the ultra-sonic frequencies - anything above the Nyquist limit (and even approaching it to a lesser degree) shows up as distortion on the final master (not necessarily noise as in white noise, but as something incorrectly quantized)... but since we're talking about ultra-highs here, and sound pressure drops 3dB per octave as you go up, those sounds are already at and beyond the limits of human hearing, and are at 16dB or so below a reference level at middle C - so I'd say it's primarily academic.
Bren R.
